A Tour of Joshua Tree National Park

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A Tour of Joshua Tree National Park

Joshua Tree, stone and tree
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Joshua Tree National Park is a wonderland of desert geology and desert geography. The photos in this tour were taken in early spring.

The rock-walled Lost Horse Valley hosts the Mojave Desert's finest grove of Joshua trees, a species of yucca, Yucca brevifolia (short-leafed yucca), with a treelike form. The leaves are stiff and extremely sharp. The Cahuilla tribe called it humwichawa.

The Mormon pioneers named the tree after the Bible's book of Joshua. It's an interesting part of the Bible, Joshua 8:18–29. Joshua led a Hebrew army in a deadly attack on the desert city of Ai following the Lord's command: "Stretch out the javelin that is in your hand toward Ai; for I will give it into your hand." Joshua's men took Ai and burned it to ruins, threw the king's body down at the city gate, "and raised over it a great heap of stones, which stands there to this day."

In fact, the book of Joshua has stones prominently featured throughout it. So the name "Joshua tree" surely derives from the stony landscape of the Mojave as well as the trees' outstretched arms and javelin-tip leaves.

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Granite of Joshua Tree National Park

The stuff of crags
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Granite is the park's signature stone. A light-colored plutonic rock formed in deep intrusions, granite underlies California's eastern mountain ranges from the Sierra Nevada to the Mexican border. This is the medium that erosion has sculpted to produce Joshua Tree's remarkable landforms.

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Boulder Pile in Granite

Inselberg in granite
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Features like this dot the landscape everywhere in the park: bedrock boulders, lying in piles. You might call them inselbergs. An inselberg ("island mountain" in German) is a bedrock knob or hill sitting in the middle of a flat erosional landscape. If only these boulder piles, instead of falling apart, had stuck together. We'll see some more pictures that shed light on why they didn't.

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Rampart of Hidden Valley

Climber's destination
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Hidden Valley is a highlight of Joshua Tree National Park: a small pocket valley, ringed with rocky walls like this granite rampart. These are prime ground for climbers because of the sound rock and abundant handholds and footholds. Notice the strong fabric of vertical and horizontal jointing here.

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Lost Horse Valley

A desert tor
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Lost Horse Valley in many respects is the centerpiece of the park. It features the best stands of Joshua trees plus many haunting landforms of the clean granite found in the park's western half. Being in this space, you can't help but feel like you've entered an old Western movie.

This isolated bedrock hill with its rounded forms qualifies as a tor. Notice the strong jointing. Joints form in rocks like this as erosion brings them closer to the surface and chemical weathering attacks them.

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How the Boulders Form

A revealing exposure
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

This hoodoo rock near Arch Rock shows how the great boulders of the park are formed. It's a long ridge of solid rock, rising slightly to the right. Picture it deeply covered with soil, many centuries ago. Groundwater softened and altered the bedrock through chemical weathering. The minerals of the granite turned to clay minerals and quartz sand. This process acts strongly on joints, tending to open them up and widen them. Rocks exposed at the surface weather into blocks by a different method.

The buried ridge was eroded more deeply on its higher, more exposed parts, so that the massive granite along its top was cut off into free-floating boulders. When the climate changed to today's desert conditions, the sand and clay was eroded away and the free boulder on the right side settled down where it sits today. Sometimes such a boulder is left improbably balanced atop another.

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Boulder Sea

A sea of boulders
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

When the underground weathering processes I described affect a whole landscape, the result can look like this sea of granite boulders near Arch Rock.

The Pinto Mountains rise in the background, marking the eastern half of the park. They are not granite, but gneiss, and they were not subjected to the same deep burial. We'll be going in that direction, but first some geological cheesecake . . .

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Adams' Tor

In the master's steps
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Ansel Adams photographed this formation in 1942. By sheer coincidence, I stood in practically the same spot in 2000.

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Desert Pavement

Rough floor for prickly furniture
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

The Cholla Garden, on the road to Pinto Basin, is a fine place to stop and stroll. The official attraction is the dense assemblage of jumping cholla ("choya"), but the geological traveler should also look down. The ground around the plants is covered with stones, not the stereotype sand. This stony covering is called desert pavement.

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Pinto Mountains Bajada

Foothill sediment wedge
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

This photograph is level, as you can tell from the ridgeline of the Pinto Mountains. The foreground is part of the sloping bajada at the base of the range. A bajada is a wide apron of rock and debris that forms where many alluvial fans coalesce. Each of the small, steep valleys in those mountains opens out into its own fan, built by flash floods carrying large amounts of sediment weathered from the valley slopes.

The Pinto Mountains are made of dark gneiss rather than the light granite of the western part of the park. At the same time, the elevation and ecosystem have changed; we have left the Mojave Desert and entered the lower, hotter Sonoran Desert. Here the dominant plant is creosote bush.

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Pinto Gneiss

Gneiss with cactus
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)
This steep slope of Pinto Gneiss, high above the roadway in the central part of the park, supports a young barrel cactus. The dark rock wall behind it shows the typical tight layering of gneiss. The rock's dark shade here is due to desert varnish. The cactus, Ferocactus cylindraceus, is intricately colored and thickly studded with wicked spines. See it and more plants of the Sonoran Desert in the large gallery of cactuses and their flowers by Judy Henning, the About.com Guide to Phoenix.
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Dry Wash

Grows where water goes
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

There is water to be found in Joshua Tree, but nearly all of it is underground. Washes like this one carry water only after it rains, and the rest of the year their sandy soil is nearly barren. The smoke tree, Dalea spinosa, is another Sonoran Desert species that favors washes. Usually the tree is leafless, conserving water, but in early spring these trees have tiny gray leaves. The twigs carry out photosynthesis the year round.

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Pinto Basin

High desert margin
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

This part of the park is the Pinto Basin, a large flat-floored valley with large mountain ranges around it. It's a very different sort of country than the western part of the park, reflecting different geology. This is the edge of the Basin and Range province, where the Earth's crust is stretched apart by the interactions of the crustal plates. The brittle rocks of the upper crust break into blocks, which sink and tilt in the hotter, softer material beneath. The parts that tilt up are the ranges, while the parts that tilt down quickly fill with sediment and become the basins. The land looks like this from here across all of Nevada and into Oregon, Idaho and Utah.

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Boulder Pile in the Pinto Basin

A gneiss pile
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

Here's another boulder pile, this time made of Pinto Gneiss. Whereas granite is a rock with little or no grain, gneiss is strongly layered. It develops jointing differently, and therefore the boulder piles look different too. These boulders are more strongly colored by rock varnish, too, perhaps because of longer exposure or the more metal-rich mineral composition.

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Pinto Basin and the Playa

The desert's extreme
(c) 2000 Andrew Alden, licensed to About.com (fair use policy)

As you move away from the mountains, the boulder piles disappear and alluvial sediments like these predominate. A longtime park ranger at Joshua Tree witnessed a rare period of alluvial activity in the Pinto Basin: "I was out there on one of our road patrols when a tremendous storm came up. I was partway across the basin on the dirt road leading to the mining district to the north, [and] it rained so hard that I stopped the car to wait out and observe the storm. The surrounding desert rapidly became a shallow 'lake' with the creosote bush, etc. sticking up out of, and reflected in, the standing water. After a brief time that entire thin sheet of water started to flow en masse down the gentle slope toward the wash in the bottom of the basin."

Thousands of years of such floods carry the fist-sized rocks seen here many kilometers into the basin but inevitably the stones are winnowed out, yielding to gravel, then sand, then silt and finally dust. In the distance is the low point of the basin—a playa or hard-baked dusty flat where almost nothing grows. The afternoon winds have whipped the dust into a choking haze that not even the creosote bush can withstand. And this is as close to it as I want to go today.

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Alden, Andrew. "A Tour of Joshua Tree National Park." ThoughtCo, Feb. 28, 2017, thoughtco.com/tour-of-joshua-tree-national-park-1440646. Alden, Andrew. (2017, February 28). A Tour of Joshua Tree National Park. Retrieved from https://www.thoughtco.com/tour-of-joshua-tree-national-park-1440646 Alden, Andrew. "A Tour of Joshua Tree National Park." ThoughtCo. https://www.thoughtco.com/tour-of-joshua-tree-national-park-1440646 (accessed November 23, 2017).